Conventional optical semiconductor devices including an optical semiconductor element mounted therein have a configuration including a metallic lead frame 1, an optical semiconductor element 2 mounted thereon, and a reflector 3 for light reflection formed from a resinous material so as to surround the optical semiconductor element 2, as shown in FIG. 1. The space including the optical semiconductor element 2 surrounded by the reflector 3 is encapsulated with a transparent resin such as a silicone resin. In FIG. 1, numeral 4 denotes a bonding wire which electrically connects an electrode circuit (not shown) formed on the metallic lead frame 1 to the optical semiconductor element 2, the bonding wire being disposed according to need.
When such an optical semiconductor device is produced, the reflector 3 is formed by the injection molding of a thermoplastic resin represented by a polyphthalamide resin (PPA) or the like. A white pigment is generally incorporated into the thermoplastic resin to reflect the light emitted by the optical semiconductor element 2 (see patent document 1).
On the other hand, when high heat resistance is required, a ceramic material containing sintered alumina is mainly used to form a part corresponding to the reflector 3 (see patent document 2). There are cases where the formation of a part corresponding to the reflector 3 from such a ceramic material is problematic in view of the suitability for mass production and cost of such packages, etc. Because of this, the reflector 3 is generally formed using a thermoplastic resin as described above.
In the case where thermoplastic resins are used as materials for forming the reflector 3, the following problems arise. Recently, the solder materials for use in mounting are becoming lead-free solder materials, which have higher melting points. As a result, the thermoplastic resins have come to be required to have thermal resistance to high-temperature reflow environments in surface mount packages such as the optical semiconductor device. Consequently, although there are desires for thermal deformation resistance at soldering temperatures in package production and for longer-period heat resistance which is required due to increases in the power of optical semiconductor elements 2, use of the thermoplastic resins results in discoloration, etc. at high temperatures and this poses problems such as a decrease in the efficiency of light reflection.
When a thermosetting resin is used as a material for forming the reflector 3 in order to overcome those problems, it is possible to obtain excellent thermal discoloration resistance and satisfactory light-reflecting properties.    Patent Document 1: JP-A-2002-283498    Patent Document 2: JP-A-2002-232017